Soft start devices are employed in compressed air systems to supply functional units of the type likely to be damaged by pressure surges, as for example servicing device or the like, with compressed air, the pressure gradually rising from a relatively low secondary pressure to the primary pressure or operational pressure. Accordingly pressure surges at a high, destructive primary pressure are prevented. Functional units liable to such pressure surge damage are for example filter units or double acting pneumatic cylinders. In the case of double acting pneumatic cylinders there may be the problem that the piston in the “pressure-less” state of the cylinder is in a middle position so that, if the full pressure surge were to act on the piston, same might be driven with an impact into the one of the end positions, something which might entail damage to the piston or at the terminal abutment in the cylinder. Dangerous movements might more particularly involve injury to persons. This is something to be prevented by a soft start so that the piston travels relatively slowly into its terminal position.
A soft start device of the type initially mentioned is for example described in the European patent publication 0 758 063 B1, in the case of which a starting valve in the form of a valve with a seat is disclosed, said valve being vented by way of an instant venting means. The starting valve possesses a housing in which a single flow path is defined extending from the inlet to the outlet, the flow path having a valve with a seat arranged on it acting as a choke.
In compressed air systems certain safety aspects must be taken into account. They are for example categorized in the German Industrial Standard DIN EN ISO 13849-1. In order to comply with the category 3 in the Standard EN 954-1 there is requirement for the compressed air device to have a so-called “single error safety” for safety-relevant functions. This means that despite a single error in the system venting is still possible.